The invention relates to a color display tube comprising in an evacuated envelope means to generate a number of electron beams and comprising an at least substantially flat display screen having areas luminescing in different colors and a color selection electrode having apertures for passing the electron beams and associating each electron beam with luminescent areas of one color, said color selection electrode being tensioned on a supporting frame of at least one set of oppositely located sides.
The invention also relates to a color selection electrode tensioned on a supporting frame for use in a colour display tube according to the invention.
Nowadays, nearly all commercial color display tubes have domed display screens. However, it is desired to provide a tube having a generally flat display screen. There are problems which have to be solved before a tube having a flat display screen is commercially available. A major problem concerns the color selection electrode. In known constructions of a tube having a domed display screen the color selection electrode is similarly curved and in such a manner that the color selection electrode varies slightly parallel to the contor of the display screen. Starting from such a known construction the color selection electrode of a tube having a substantially flat display screen should have a flat contor. However, such a color selection electrode has an insufficient non-deformability or rigidity. One manner of giving a color selection electrode rigidity is disclosed in U.S. Pat. No. 4,069,567. In this specification a method is disclosed of installing a color selection electrode in a color display tube in which the color selection electrode is kept tensioned in a supporting frame. The supporting frame may be of metal and may be placed in the evacuated envelope of The tube at a certain distance from the display screen. Alternatively the supporting frame may be substantially of glass, so that, for example, the frame may be constituted by the display screen or the evacuated envelope of the tube. In a preferred embodiment of this method of installing a color selection electrode, the color selection electrode is manufactured from a material which has a coefficient of thermal expansion exceeding that of the supporting frame. The color selection electrode and the supporting frame are heated together, for example in an oven, while the color selection electrode is tensioned. Simultaneously the color selection electrode is heated complementarily. The supporting frame and the color selection electrode consequently expand. However, as a result of the complementary heating the color selection electrode expands more than the supporting frame. The color selection electrode is then fixed to the supporting frame. The color selection electrode and the supporting frame are finally cooled to room temperature as a result of which the color selection electrode is extra tensioned.
During the processing and the operation of the display tube, however, temperature differences occur between the color selection electrode and the supporting frame which can increase or decrease the tension of the color selection electrode. During the cooling portion of a number of process steps, for example bonding components of the envelope together and evacuating the display tube, the supporting frame will be warmer than the color selection electrode as a result of the difference in thermal capacity. This causes such a great difference in expansion between the supporting frame and the tensioned color selection electrode that the tension in the color selection electrode in this stage can become larger than the elastic proof stress (i.e. the elastic limit) of the color selection electrode, as a result of which it may be deformed permanently. After cooling the adhered or evacuated display tube the deformed color selection electrode is slack in the supporting frame. As a consequence mislanding occurs, i.e. each electron beam is not properly associated with luminescent areas of one color.
It is an object of the invention to provide a color display tube having a color selection electrode which is secured to a supporting frame with a tension which is as large as possible, in which differences in thermal expansion between the color selection electrode and the supporting frame can be permitted without the tension in the color selection electrode becoming larger than the elastic proof stress of the color selection electrode.